Throughout the game, never kill any enemies or gain any EXP/LV; essentially, only use non-lethal battle exits (spare, flee, or battle-ending ACTs). FIGHTing monsters, killing the Dummy in the Ruins, or attacking Mad Mew Mew when she can be spared does not cause this route to be aborted, as they do not accrue EXP. If one accidentally kills a monster, they can simply reload their save file.
After fighting Papyrus, visit his house and complete the date/hangout with him.
After Undyne passes out from pursuing the protagonist, get a cup of water from the nearby water cooler and pour it on her.
Leaving the room while she is still passed out aborts the True Pacifist Route, as she disappears from her spot when coming back. Papyrus is in front of her house and tells the protagonist that she got heat stroke, and they cannot hang out with her.
Return to Waterfall and go to Undyne's House. Papyrus is waiting outside. Talk to him to enter the house, then complete the "cooking lesson" and faux battle with Undyne.
Go through Hotland and the CORE. In Mettaton EX's fight, get the ratings to 10,000 (this includes using Alphys's phone upgrade to shoot at his heart during some of his combat rounds) to end the battle peacefully.
Defeat Asgore and Photoshop Flowey, then reopen the game (if the Neutral Route has been completed on a previous playthrough, this step can be skipped). The next series of events can be triggered in two ways, provided that no EXP/LV have been gained: either by going through the game normally from the beginning after completing a Neutral Route, or by reloading the final SAVE file of a Neutral Route.
Backtrack out of the CORE towards the MTT Resort, to the bridge connecting the two places. There, Undyne calls the protagonist, asking to deliver a letter for her.
Talk to Undyne outside of Papyrus's house to receive Undyne's Letter.
If the protagonist had reached the bridge without completing the hangout with Undyne, then backtracked to complete such, she gives them the letter after the hangout, unless their inventory is full, in which case they have to receive it from her at Papyrus's house.
If the protagonist discards the letter, they can return to Undyne to receive Undyne's Letter EX, which cannot be discarded.
Put the letter under the door of Alphys's Lab and complete the "date" with her.
Upon leaving the Garbage Dump (where befriending Alphys ends), Papyrus calls and tells the protagonist to meet Alphys at her lab. Return to Alphys's Lab to find a note leading to her "bathroom", which is actually an elevator leading to the True Lab. Upon entering the True Lab, the True Pacifist Route cannot be aborted.
Find the four keys scattered throughout the True Lab and put them in their proper slots to open the generator room. Turn on the generator to trigger a conversation with Alphys. Head back to the elevator near the generator room to be warped to New Home, with the exit blocked off by vines. This signals that the True Pacifist Ending will occur. It is impossible to leave New Home after this point.